1. Field of the Invention
The present invention relates to an adhesive which exhibits good adhesion properties when applied to metals, especially precious metals. The term "adhesive" used in the present invention comprehends primers which are used when a metallic material is bonded to other materials. The adhesive of the present invention is useful particularly in the field of dentistry.
2. Description of the Prior Art
The adhesive of polymerization-curing type composed of polymerizable monomers such as acrylic monomers and epoxy compounds is in general use for the bonding of metallic materials because of its good workability and its ability to cure in a short time at normal temperature under normal pressure. It plays an important role in the field of adhesive. However, it suffers from a serious disadvantage in some application areas where water resistance is required, because it rapidly decreases in bond strength when its bonding interface is exposed to water at all times.
In the field of dental materials, attempts have recently been made to obtain an adhesive of polymerization-curing type which has a high bond strength for metals and good water resistance. These attempts are bearing fruit. An example is the dental adhesive incorporated with an organophosphate ester (as disclosed in U.S. Pat. No. 4,539,382). It produces a high bond strength which is very little affected by water when applied to base metals such as iron, nickel, chromium, cobalt, tin, aluminum, copper, and titanium, and alloys thereof. It is now in practical use as a dental adhesive. Unfortunately, when it is used for dental casting (e.g., inlay, crown, and bridge) of precious metal alloys (e.g., gold, platinum, palladium, and silver alloys), the bond strength is a little more affected by water than when it is used for base metal alloys. To keep the bond strength unaffected by water, it was necessary that the adherend of precious metal alloys undergo surface treatment such as tin plating and oxidation.
It has recently been found that the bond strength for precious metals becomes moderately unaffected by water when an MMA-tributyl borane adhesive is applied to a precious metal adherend primed with N-(4-mercaptophenyl)methacrylamide. This was reported in The Journal of the Japanese Society for Dental Materials and Devices, vol. 5, p. 92-105 (1986).